1. Field of the Invention
The present invention relates to a printing apparatus which performs multipath printing of scanning a plurality of number of times the same printing area of a printing medium with a printhead having a plurality of orifices arranged in a printing medium convey direction, thereby forming an image based on input image information on the printing medium, a control method therefor, and a computer-readable memory.
2. Description of the Related Art
In resent years, OA devices such as personal computers, copying machines, and wordprocessors have become popular. As a kind of printing apparatus for these devices, inkjet printing apparatuses for printing an image by an inkjet printing method are rapidly developed and popularized. With advanced functions of OA devices, color images are required, and various color inkjet printing apparatuses are being developed.
In general, the inkjet printing apparatus comprises a printing means (printhead), a carriage for mounting an ink tank, a convey means for conveying a printing sheet, and a control means for controlling them. A printhead for discharging ink droplets from a plurality of orifices is scanned in a direction (to be referred to as a main scanning direction) perpendicular to the convey direction (to be referred to as a subscanning direction) of a printing sheet. In non-printing, a printing sheet is intermittently conveyed along said subscanning direction by an amount equal to a printing width. A color inkjet printing apparatus forms a color image by overlapping ink droplets discharged from printheads of a plurality of colors on a printing medium.
Examples of the method of printing an image by discharging ink in the inkjet printing apparatus are a method using an electrothermal energy converter in which a heating element (electrothermal energy converter) is disposed near an orifice and an electrical signal is applied to the heating element to locally heat ink and change the pressure, thereby discharging ink from the orifice, and a method using an electro-mechanical converter such as a piezoelectric element. A known example of the means of discharging ink is an arrangement using an electro-pressure conversion means, such as a piezoelectric element, to apply a mechanical pressure to ink, thereby discharging the ink.
These methods print characters and figures by discharging small ink droplets from an orifice onto a printing medium in accordance with print data. The inkjet printing apparatus hardly generates noise because of a non-impact type, can reduce the running cost and apparatus size, and can relatively easily print a color image. With these advantages, the inkjet printing apparatus is employed in a computer, wordprocessor, and the like. Further, the inkjet printing apparatus is widely used as a printing apparatus mounted on a stand-alone copying machine, printer, facsimile, and the like.
In the printing method of the conventional inkjet printing apparatus, a dedicated coated sheet having an ink absorption layer must be used to obtain a high-development color image free from any ink blur on a printing medium. Recent improvements of ink and the like allow practically using a method having printability on plain sheets which are enormously consumed in a printer, copying machine, and the like. In addition, demands arise to cope with various printing media having different ink absorption characteristics, such as an OHP sheet, cloth, and plastic sheet. To meet these demands, printing apparatuses capable of performing best printing regardless of the type of printing medium are being developed and put into practical use. As for the size of a printing medium, demands arise for printing on a large-size printing medium such as printing on an advertising poster, cloth such as clothes, and the like. Such inkjet printing apparatus is being demanded as an excellent printing means in various industrial fields. Higher image qualities and higher speeds are also being required.
In general, the printing method of the color inkjet printing apparatus realizes color printing using three, cyan (Cy), magenta (Mg), and yellow (Ye) color inks or four color inks including a black (Bk) ink. This color inkjet printing apparatus prints a color image, unlike a monochrome inkjet printing apparatus mainly used to print characters, and is required for various factors such as the color development, gradation, and uniformity of an image to be printed.
However, the quality of an image to be printed greatly depends on the performance of the printhead itself. That is, slight differences between orifices caused in manufacturing the printhead, such as variations in shapes of the orifices of the printhead or electrothermal converters (discharge heaters), influence the discharge amount and direction of discharged ink, resulting in low image quality as density nonuniformity of a final printed image. Consequently, a xe2x80x9cblankxe2x80x9d portion which inhibits an area factor of 100% periodically appears in the main scanning direction, dots excessively overlap each other, or a blank stripe appears on a printing medium. These phenomena are sensed as density nonuniformity by a human eye.
To prevent this density nonuniformity, a multipath printing method is proposed. This multipath printing method will be described with reference to FIG. 11.
In FIG. 11, a multipath printing method using a printhead of a single ink color having eight nozzles (orifices) will be exemplified for descriptive convenience.
FIG. 11 is a view for explaining the multipath printing method.
In the first scanning of the printhead in the main scanning direction, a staggered pattern (a pattern to be printed at xe2x97xaf positions) is printed using first four nozzles out of the eight nozzles of the printhead. The printing sheet is fed in the subscanning direction by half the printing width of the printhead (by a width of 4 dots in this case). Then, in the second scanning of the printhead, an inverted staggered pattern (a pattern to be printed at ◯ positions) is printed using all the eight nozzles of the printhead to complete printing in a printing area corresponding to half the printing width of the printhead. That is, a 4-dot wide printing area is completed every scanning by sequentially feeding the printing sheet in units of 4 dots and alternately printing staggered and inverted staggered patterns. In this way, one line (printing area by one scanning with the printing width of the printhead) is printed using two different nozzles, thereby forming a high-quality image almost free from density nonuniformity. Also, the multipath printing method can perform printing while drying ink.
Known examples of a method of generating data (path data) not to be printed (not to discharge ink) in each scanning are a method (fixed thinning method) of generating path data by thinning out print data using a staggered/inverted staggered pattern, as described above, a method (random thinning method) of generating path data by thinning out print data using a random mask pattern prepared by laying out print dots and non-print dots at random, and a method (data thinning method) of generating path data by thinning out print dots.
If the printhead of the inkjet printing apparatus does not discharge any ink for a long time, the ink viscosity increases in the ink passage near the orifice, failing in normal discharge. If printing operation having a high print dot ratio (high printing duty) continues, small bubbles are produced in ink in the ink passage along with ink discharge, and grow and remain in the ink passage to influence discharge, failing in normal discharge. In addition to these bubbles produced with ink discharge, bubbles may be mixed in ink in an ink supply system such as the interconnection of an ink supply path.
This ink discharge error degrades reliability of the printing apparatus. If printing continues in an erroneous discharge state, the printhead temperature rises to a higher level than a normal level to damage the printhead itself and deteriorate the durability.
To avoid an ink discharge error caused by various factors, the inkjet printing apparatus adopts recovery processing such as {circle around (1)} capping processing of capping the orifice surface of the printhead in a non-discharge state to prevent an increase in ink viscosity, {circle around (2)} ink suction processing of sucking ink from the orifice in a capped state to forcibly discharge high-viscosity ink, and {circle around (3)} pre-discharge processing of discharging ink to a predetermined ink receiver formed from an ink absorber or the like to discharge high-viscosity ink.
The inkjet printing apparatus preferably comprises a detection means for detecting an ink discharge error in association with recovery processing in order to improve the reliability of the printing apparatus. The detection means preferably individually detects a discharge error for a plurality of orifices of the printhead. As the detection means, various methods are employed: a method using a photosensor disposed on an ink flying path, a method of determining an ink discharge error by a temperature rise and subsequent temperature drop of the printhead caused by no-ink discharge, and a method of detecting an ink discharge error by printing a predetermined test pattern on a printing medium and reading out the printed pattern.
However, the conventional printing apparatus suffers the following problems in association with measures against an ink discharge error.
When an ink discharge error is detected during page printing, recovery processing is generally immediately executed. Recovery processing is done while interrupting printing processing, which prolongs the total printing processing time. In an inkjet printing apparatus for performing multipath printing, the time interval between a plurality of printing scanning operations for the same printing area becomes longer than normal printing scanning by a time required for recovery processing. In general, a subsequently landed ink droplet penetrates in directions perpendicular to and along the sheet surface, but rarely penetrates and fixes in an area where a previously landed ink droplet has penetrated. The subsequent ink droplet penetrates and fixes below the area where the previous ink droplet has penetrated. However, when a next ink droplet is landed a long time interval after previous printing scanning, the next ink droplet penetrates by a relatively large amount in an area where a previous ink droplet has penetrated and fixed. Thus, the printing density becomes high when the time interval of printing scanning is long. As a result, the density may become nonuniform in an area where the time interval of printing is long due to recovery processing. In addition, ink suction processing particularly consumes a large amount of ink, so it is not preferable to frequently perform recovery processing.
If normal ink discharge cannot be resumed even by recovery processing and the printhead fails, the printhead must be exchanged with a new one capable of normal operation. Until the printhead is exchanged, density nonuniformity or a blank stripe caused by an ink discharge error must be permitted, or the printing function itself must be completely stopped.
To solve these problems arising from an ink discharge error, the printhead may be equipped in advance with, e.g., a nozzle not used in normal printing, and when a non-discharge nozzle is detected, the prepared complementary nozzle complementarily prints instead of the non-discharge nozzle. However, a spare nozzle provided to the printhead inhibits downsizing of the printhead and increases the cost of the printhead as expendables.
The present invention has been made to eliminate the above drawbacks, and has as its object to provide a printing apparatus capable of suppressing an increase in total printing time caused by recovery processing during printing and forming a high-quality image free from density nonuniformity, a control method therefor, and a computer-readable memory.
To achieve the above object, a printing apparatus according to the present invention comprises the following arrangement.
That is, a printing apparatus which performs multipath printing of scanning a plurality of number of times the same printing area of a printing medium with a printhead having a plurality of orifices arranged in a printing medium convey direction, thereby completing an image based on input image information on the printing medium, comprises generation means for generating a mask table for outputting print data corresponding to each of a plurality of scanning operations, detection means for detecting faults of the plurality of orifices, change means for changing contents of the mask table, generated by the generation means, on the basis of a detection result of the detection means, and output means for outputting print data based on the image information from each of the plurality of scanning operations using the mask table generated by the generation means or the mask table changed by the change means.
To achieve the above object, a printing apparatus control method according to the present invention comprises the following steps.
That is, a control method for a printing apparatus which performs multipath printing of scanning a plurality of number of times the same printing area of a printing medium with a printhead having a plurality of orifices arranged in a printing medium convey direction, thereby completing an image based on input image information on the printing medium, comprises the generation step of generating a mask table for outputting print data corresponding to each of a plurality of scanning operations, the detection step of detecting faults of the plurality of orifices, the change step of changing contents of the mask table, generated in the generation step, on the basis of a detection result in the detection step, and the output step of outputting print data based on the image information for each of the plurality of scanning operations using the mask table generated in the generation step or the mask table changed in the change step.
To achieve the above object, a computer-readable memory according to the present invention comprises the following program codes.
That is, a computer-readable memory storing control program codes for a printing apparatus which performs multipath printing of scanning a plurality of number of times the same printing area of a printing medium with a printhead having a plurality of orifices arranged in a printing medium convey direction, thereby completing an image based on input image information on the printing medium, comprises a program code of the generation step of generating a mask table for outputting print data corresponding to each of a plurality of scanning operations, a program code of the detection step of detecting faults of the plurality of orifices, a program code of the change step of changing contents of the mask table, generated in the generation step, on the basis of a detection result in the detection step, and a program code of the output step of outputting print data based on the image information for each of the plurality of scanning operations using the mask table generated in the generation step or the mask table changed in the change step.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.